First, make sure that no other app is using your microphone. This includes any other app that records audio.

A small percentage of devices (especially lower cost and older ones) do not have the system resources needed to continuously listen for speech.  This may be a temporary issue, which can be solved with a reboot, or it may be permanent.  Try rebooting your device and starting Ultimate Driving Mode as soon as the reboot is done.  If the problem occurs again, then your device is most likely incapable of listening continuously for the trigger.  To work around this, visit the Trigger Phrases section of the settings and make an adjustment so that the app only listens for a command after a screen tap.  This may allow you to still use the app.